There have been widespread celebrations in Romania and Bulgaria to mark their accession to the EU.
They bring 30 million new members to the union, and expand the number of members to 27.
Most of the 15 older member states have put in place restrictions on the free movement of workers from the two new members - although Finland and Sweden are two exceptions.
Tens of thousands of people celebrated at midnight at concerts in the two capitals, Sofia and Bucharest.
The Romanian President, Traian Basescu, said EU entry was an 'enormous chance for future generations' while Bulgaria's leader said it was a 'heavenly moment'.
Romania's foreign minister, Mihai Razvan Ungureanu, said his country had a lot to contribute to the EU.
Meanwhile, Slovenia has become the latest member of the Eurozone, which became the country's currency at midnight.
The Slovenian currency, the tolar, will remain in use alongside the euro for 14 days.
